1. Field of the Invention
The present invention relates to a variant tax gene of bovine leukemia virus. The variant tax gene has enhancing ability to induce bovine leukemia virus replication.
2. Disclosure of the Related Art
Bovine leukemia virus (BLV) is a retrovirus most closely related to human T-cell leukemia virus (HTLV). Similar to human immunodeficiency virus (HIV), this virus has been known to have a gene that regulates viral replication and also tax gene that participates in transcription. The ratio of cattle infected by the virus (infection rate in Japan) is 10 to 20%, and 1 to 2% of the infected cattle develop extremely malignant enzootic bovine leukosis to die after a long latent period of about 10 to 15 years. Economic loss of stockbreeders caused by the virus is very serious, and accordingly, researches of the mechanism of the replication of BLV are highly important to develop means for preventing or treating infection caused by the virus.
The inventors of the present invention noted the fact that bovine individuals infected by BLV are classified into three characteristic pathological conditions, i.e., healthy individuals but positive to the antiviral antibody, those developing persistent lymphocytosis (PL) and those developing enzootic bovine leukosis, which is B lymphoma, after a long latent period, and the inventors made efforts to elucidate cause of the phenomenon at a genetic level. As a result, they found that various variants of the tax gene exist, and virus strains with a Tax protein including one or more particular mutations had enhanced transcription activity compared to wild-type virus strains.
The present inventors further continued researches, and found that mutations of the tax gene effected not only on the ability to activate the expression of the viral gene, but also on the strength of viral proliferation ability, and that viral strains having a particular variant tax gene had remarkably enhanced proliferation ability. The present invention was achieved on the basis of these findings.
The present invention thus provides a variant tax gene of bovine leukemia virus, which encodes a variant gene product containing one or more mutations selected from the group consisting of:
substitution of the 240th serine for threonine,
substitution of the 247th aspartic acid for glycine,
substitution of the 251st threonine for alanine,
substitution of the 258th aspartic acid for glycine,
substitution of the 261st histidine for arginine,
substitution of the 261st histidine for tyrosine, and
substitution of the 265th serine for glycine.
The present invention also provides a gene which contains the aforementioned variant tax gene and functions to enhance ability to induce replication of the bovine leukemia virus or a retrovirus related to the bovine leukemia virus (e.g., HTLV and HIV), preferably the bovine leukemia virus; and a gene which contains the aforementioned variant tax gene and functions to enhance ability to activate transcription of the bovine leukemia virus or a retrovirus related to the bovine leukemia virus, preferably the bovine leukemia virus.
According to another aspect of the present invention, there is provided a protein which is a product of a bovine leukemia virus tax gene and contains one or more mutations selected from the group consisting of:
substitution of the 240th serine for threonine,
substitution of the 247th aspartic acid for glycine,
substitution of the 251st threonine for alanine,
substitution of the 258th aspartic acid for glycine,
substitution of the 261st histidine for arginine,
substitution of the 261st histidine for tyrosine, and
substitution of the 265th serine for glycine.
The present invention also provides the aforementioned protein which enhances ability to induce replication of bovine leukemia virus or a retrovirus related to the bovine leukemia virus, preferably the bovine leukemia virus; and the aforementioned protein which enhances ability to activate transcription activity of bovine leukemia virus or a retrovirus related to the bovine leukemia virus, preferably the bovine leukemia virus.
According to preferred embodiments of the present invention, the aforementioned variant gene product is selected from those including substitution of the 240th amino acid, i.e., serine, for threonine, substitution of the 247th amino acid, i.e., aspartic acid, for glycine, or substitution of the 261st amino acid, i.e., histidine, for arginine. The gene of the present invention preferably encodes any one of the aforementioned preferred gene products.
According to another aspect of the present invention, there is provided a bovine leukemia virus having enhanced replication ability which contains the aforementioned gene. According to still another aspect of the present invention, there is provided a recombinant vector containing the aforementioned gene of the present invention which has enhancing ability to induce replication of bovine leukemia virus or a retrovirus related to the bovine leukemia virus, preferably the bovine leukemia virus.
According to still further aspects of the present invention, there are provided a method for producing a wild-type bovine leukemia virus, which comprises the steps of infecting a host cell introduced with a recombinant vector containing the aforementioned gene with a wild-type bovine leukemia virus, and culturing the resulting cell; and a method for enhancing ability to induce replication of bovine leukemia virus or a retrovirus related to the bovine leukemia virus, preferably a wild-type bovine leukemia virus, which comprises the step of expressing the aforementioned gene in a cell infected by the bovine leukemia virus or the retrovirus related to the bovine leukemia virus.